The present invention generally relates to electronic circuits, and more particularly to circuit protection devices comprising a resistor with a positive temperature coefficient (PTC), referred to as PTC_r, operating in combination with a semiconductor device.
PTC_r circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions, is in a low temperature, low resistance state. However, if the current through the PTC_r device increases excessively, and/or the ambient temperature around the PTC_r device increases excessively, then the PTC_r device will be "tripped," i.e., converted to a high resistance state such that the current is reduced substantially to a safe level. Generally, the PTC_r device will remain in the tripped state, even if the fault is removed, until the device has been disconnected from the power source and allowed to cool. After the current and/or temperature return to their normal levels, the PTC_r device will switch back to the low temperature, low resistance state.
An example of a PTC_r composition device is one that contains a PTC_r which is composed of a PTC conductive polymer. The largest steady state current which will not cause any of the devices in a batch of devices to trip is referred to as the "hold current" (I.sub.hold), and the smallest steady state current which will cause all of the devices to trip is referred to as the "trip current" (I.sub.trip). In general, the difference between I.sub.hold and I.sub.trip decreases slowly as the ambient temperature increases, and the higher the ambient temperature, the lower the hold current and the trip current.
PTC and semiconductor devices have been used together in electrical circuits. In some instances the PTC_r devices have been used to protect the semiconductor devices from overcurrent and/or overtemperature conditions. In other instances, the semiconductor and PTC_r devices have been used together to protect the circuits in which they reside from overcurrent and/or overvoltage conditions.
In many instances, it is desirable to use PTC_r devices in high voltage applications. However, in most cases, the PTC_r device that is operated at its normal current generally has a large size. PTC_r devices of large size are not suitable for portable electronic devices in which size and weight are important. PTC_r devices of large size are also not suitable for high density electronics circuits.